In offshore petroleum operations, platforms comprising a trussed steel framework, or jacket, secured to the sea floor and a deck mounted on top of the jacket are commonly used to drill for and produce oil and gas. Such an offshore structure will, by the nature of its fabrication, contain a considerable number of welded steel joints located in highly stressed regions. These welds may contain small zones of low fracture toughness, otherwise known as local brittle zones ("LBZs"), that may cause a local susceptibility to brittle fracturing. Industry is concerned that these LBZs may increase the probability of fracturing.
The process of welding produces HAZ in the steel adjacent to the weld metal which results from the intense heat associated with welding. FIGS. 1, 2A, 2B, and 3 are schematics of typical welds, each having a weld metal 20, base metal 22, and a HAZ 18. FIG. 1 illustrates the various regions of a typical single-pass bead-on-plate weld. As shown, with this type of weld the HAZ 18 is adjacent to the weld metal 20 and comprises coarse grain 10, fine-grain 12, intercritical 14, and subcritical 16 regions. In a typical multipass weld, as illustrated in FIG. 2B, the HAZ 18 of adjacent weld passes overlap and create additional coarse grain regions including the single thermal cycle region ("CGHAZ") 10, the two-thermal-cycle intercritically reheated region ("IRCG") 26, and the subcritically reheated region ("SRCG") 28. Low toughness behavior in the HAZ is caused by LBZs located within the HAZ. The coarse grain regions of the HAZ are a primary site for LBZs. LBZs are a direct result of welding thermal cycles that heat the base steel to a peak temperature near the melting temperature of the steel.
Although the structural significance of LBZs is not yet established in the industry, some steel users have elected to determine the HAZ toughness of candidate steels prior to purchase. Low toughness behavior is generally known to be greatest in the coarse grain regions of the HAZ. However, determining the HAZ toughness of a steel can be difficult because the LBZs in these coarse grain regions are small and discontinuous; performing quantitative toughness testing of these small regions is difficult. FIG. 3 is a schematic illustrating how LBZs 30, or low toughness coarse grain regions, might be positioned in an actual weld. LBZs are typically 0.25 to 0.5 mm thick and 1.0 to 5.0 mm high.
Various tests are commonly used in the industry to determine the HAZ toughness of various steels. One such test is the crack tip opening displacement test ("CTOD"). CTOD testing involves initiating and propagating a fatigue crack in a steel sample and subsequently testing that sample to final fracture. The resulting CTOD value represents the width of the fatigue crack tip blunting prior to failure, which characterizes the fracture toughness of the steel. CTOD values below 0.10 mm are generally considered to indicate low resistance to fracturing, or low toughness, and CTOD values above 0.25 mm are generally considered to indicate high resistance to fracturing, or high toughness. To evaluate the fracture toughness of the coarse grain zones having LBZs, the fatigue crack tip must terminate in the coarse grain region. As discussed above, these regions are small and therefore placement of the fatigue crack is difficult. Furthermore, waviness of the weld beads in the weld direction and fusion line waviness in the through thickness direction will create significant difficulties for locating the fatigue crack tip in the correct zone of interest. In addition, the fatigue crack, even if in the correct position within the HAZ, may deviate into the base or the weld metal.
Because of the problem with fatigue crack tip placement, various standards in the industry, such as the American Petroleum Institute Recommended Practice 2Z ("API RP 2Z") and the Engineering and Equipment Material Users Association Standard ("EEMUA 150") which are well known to those skilled in the art, provide that a minimum number of samples must show that the fatigue crack tip is in the coarse grain regions for a minimum percentage of the sample thickness in order to ensure the accuracy of test results. To make this determination, detailed metallographic studies must be performed on at least 30 to 60 weldment samples, and as a result, CTOD testing can take as long as six months or more to complete and can be very expensive.
The present invention is aimed at alleviating the above described problems and providing a practical method for determining the relative HAZ toughness of steel prior to purchase. It is a further aim of the invention to reduce testing time and cost so that the influence of steel chemistry and manufacturing procedures can be economically evaluated to improve the HAZ toughness.